1. Field of the Invention
The invention relates to an oxide magnetic material usable for a multilayer inductor and a laminated ceramic substrate and the like in a high frequency circuit member and the like and its production method.
2. Related Art
With the recent trend of miniaturization and use of higher frequency of electronic apparatus, a magnetic material usable in a high frequency band is more and more needed. As such a magnetic material for high frequency, NiCuZn-series spinel ferrites have been conventionally used, however in the case of frequency of several hundred MHz or higher, they cause natural resonance to result in increase of the loss and become incapable of practically working as magnetic materials. As a magnetic material usable up to a GHz band, hexagonal ferrites with high magnetic anisotropy defined as Ba3Me2Fe24O42 (Me: a bivalent metal) and the like can be exemplified. Further, in order to improve the high frequency properties by improving the anisotropy, it is tried to replace some of Ba in the above-mentioned hexagonal type ferrites with Sr.
However, in the GHz band, the imaginary component (xcexcxe2x80x3) of the magnetic permeability becomes so significant as to increase the loss for use such ferrites for inductors.
Also, in a high frequency circuit member, use of a laminated ceramic substrate comprising a magnetic ceramic substrate and a dielectric ceramic substrate laminated on each other has been tried for miniaturization. With respect to such a laminated ceramic substrate, the patterned wiring of a capacitor is formed on a dielectric ceramic substrate and the patterned wiring of an inductor is formed on a magnetic ceramic substrate.
FIG. 4 is a perspective view showing one example of such a laminated ceramic substrate and FIG. 5 is an exploded perspective view. As illustrated in FIG. 4 and FIG. 5, the laminated ceramic substrate is composed by laminating a plurality of ceramic substrates 3 and 4. A plurality of wiring patterns 11 composing inductors and capacitors are formed on the surfaces of the respective ceramic substrates 3 and 4 by a screen-printing method or the like.
In the case the ceramic substrates 3 are magnetic ceramic substrates and the ceramic substrates 4 are dielectric ceramic substrates, the wiring patterns 11 composing the inductors are formed on the magnetic ceramic substrates 3 and wiring patterns 11 composing the capacitors are formed on the dielectric ceramic substrates 4. The wiring patterns 11 between the substrates are connected through via holes 12.
After laminated, these ceramic substrates 3 and 4 are united by firing at a high temperature to obtain a laminated ceramic substrate.
In the case the wiring patterns 11 are formed by using Ag or the like with a high conductivity, it is required to carry out firing at a temperature as low as about 900xc2x0 C. If firing is carried out at a high temperature, the shape of the wiring patterns of Ag or the like is deformed to make it impossible to form desired circuits on the respective substrates.
However, a conventional magnetic ceramic material like a hexagonal ferrite or the like has a suitable firing temperature of 1,300xc2x0 C. or higher, and it has a problem that good magnetic properties cannot be obtained in the case firing is carried out at a temperature as low as about 900xc2x0 C.
It has been tried to carry out firing at a low temperature by adding a sintering aid such as B2O3, CuO, and Bi2O3, neither sufficient effect on low temperature firing has been obtained yet nor the magnetic loss has been lowered sufficiently. Especially, with respect to a hexagonal ferrite in which some of Ba""s are replaced with Sr""s, any sufficient effect has not been obtained so far.
The object of the present invention is to provide an oxide magnetic material which can be produced by firing at a low temperature and has good magnetic properties in a high frequency band and its production method.
The oxide magnetic material of the invention is a Sr-containing oxide magnetic material having grain boundary phases in crystal grains, containing not less than 2% by weight of Sr in the grain boundaries and not less than 10% by weight of at least one element selected from Bi, V, B and Cu.
With respect to the oxide magnetic material of the invention, existence of not less than 2% by weight of Sr in the grain boundaries and not less than 10% by weight of the above-mentioned elements in the material can decrease the magnetic loss and give good magnetic properties. Further, the oxide magnetic material can be produced by firing at a low temperature.
The content of Sr in the grain boundary phases is preferably not less than 2% by weight, more preferably not less than 5% by weight and its upper limit is preferably not more than 30% by weight. When the content of Sr in the grain boundary phases is less than 2% by weight, shrinkage after firing at a temperature as low as about 900xc2x0 C. is scarcely observed and a specimen after firing is unsatisfactory in the mechanical strength or the like and the magnetic loss is increased. On the other hand, if the Sr content exceeds 30% by weight, the content of other elements such as Bi and the like is relatively decreased, so that densification in the case of low temperature firing sometimes does not take place.
The content of the additive elements in the grain boundary phases is preferably not less than 10% by weight, more preferably not less than 25% by weight and its upper limit is not more than 70% by weight. If the content of additive elements is less than 10% by weight, shrinkage after firing at a temperature as low as about 900xc2x0 C. is scarcely observed and a specimen after firing is unsatisfactory in the mechanical strength or the like and the magnetic loss is increased. On the other hand, if the content of additive elements exceeds 70% by weight, the magnetic permeability (a real component) decreases in some cases.
As for the additive elements, use of Bi is especially preferred. The additive elements may be contained in an oxide magnetic material by adding oxides containing the additive elements to a preliminarily baked powder obtained by preliminarily baking a raw material powder of the oxide magnetic material and firing the obtained mixture. The oxides containing the additive elements include Bi2O3, V2O5, B2O3, CuO, and the like. The content of the additive elements in the grain boundary phases can be adjusted by adjusting the amount of the oxides to be added to a preliminarily baked powder.
In the invention, together with the oxides containing additive elements, an oxide containing Sr may be added to the preliminarily baked powder and the obtained mixture may be fired to add Sr in the oxide magnetic material. Addition of the oxide containing Sr to the preliminarily baked powder increases the content of Sr especially in grain boundary phases. The content of Sr in the grain boundary phases can be also controlled by changing the pulverizing and mixing conditions at the time of pulverizing and mixing the preliminarily baked powder and the oxide containing Sr. For example, if the mixing is carried out for a long duration, the content of Sr in the grain boundary phases can be increased. Further, the content of Sr in the grain boundary phases can be controlled by changing the temperature at the time of preliminarily baking the raw material powder of the oxide magnetic material. That is, if the preliminarily baking temperature is decreased, the content of Sr in the grain boundary phases can be increased.
In the invention, Si may be contained further in the grain boundary phases. The content of Si in the grain boundary phases is preferably not less than 2% by weight, more preferably not less than 3% by weight and its upper limit is preferably not more than 20% by weight. Existence of Si in the grain boundary phases increases the shrinkage ratio of a fired oxide magnetic material and improves the magnetic properties. If the content of Si in the grain boundary phases is less than 2% by weight, the effect of Si-coated to suppress the loss is sometimes insufficient (that is, the value of xcexcxe2x80x2/xcexcxe2x80x3 becomes low). Meanwhile, if the content of Si exceeds 20% by weight, the value of permeability (the real component) tends to be significantly decreased.
Si can be contained in the grain boundary phases by adding an oxide containing Si together with oxides containing additive elements to the preliminarily baked powder of the oxide magnetic material and firing the resulting mixture. Incidentally, it is no need for Si in the grain boundary phases to exist evenly in the grain boundary phases but Si may exist unevenly in some portions of the grain boundary phases. For example, additive elements of such as Bi may exist more in some portions of the grain boundary phases, and Si may exist more in some other portions.
In the invention, the oxide magnetic material may contain a Group Ia element or a Group IIa element of a periodic table. Practical examples of the Group Ia element and the Group IIa element of a periodic table include Ca, K, Na, Sr, and Ba. Among them, Ca is preferable especially.
A Group Ia element or a Group IIa element can be contained in the oxide magnetic material by adding an additive, a compound of the Group Ia element or the Group IIa element with a Group VIIb element of a periodic table, together with oxides containing additive elements to the preliminarily baked powder of the oxide magnetic material and firing the resulting mixture.
The crystal grains of the oxide magnetic material of the invention preferably have an average grain diameter of 0.01 xcexcm or large and 3 xcexcm or smaller. The average grain diameter of the crystal grains can be measured from a cross-sectional photograph taken by a scanning electron microscope (SEM). More practically, it can be calculated by subjecting the cross-sectional photograph of a SEM to image processing to calculate the surface areas of the respective crystal grains and calculating the respective diameters of circles from them by assuming the crystal grains to be true circles. If the average crystal grain diameter is larger than 3 xcexcm, shrinkage after firing at a temperature as low as about 900xc2x0 C. is scarcely observed and a specimen after firing is unsatisfactory in the mechanical strength or the like and the magnetic loss is increased in some cases. On the other hand, if the average crystal grain diameter is minute, smaller than 0.01 xcexcm, the crystal grains are easily agglomerated to make it difficult to obtain a slurry in which a magnetic material is evenly dispersed in some cases.
In the invention, the oxide magnetic material is preferably a hexagonal ferrite. Practically, the crystal grains are preferable to have a crystal structure of a hexagonal ferrite. It is further preferable for the hexagonal ferrite to have Z phase defined as M3Me2Fe24O41 (M denotes Ba and/or Sr; and Me denotes a bivalent metal) as a main phase. Further, the main phase is preferably defined as (SrxBa1-x)3Me2Fe24O41 (x is a value satisfying 0xe2x89xa6xxe2x89xa61).
A production method of the invention is a production method capable of producing the above-mentioned oxide magnetic material of the invention and involves steps of preparing a preliminarily baked powder by preliminarily baking a raw material powder of an oxide magnetic material, preparing a mixed powder by mixing an oxide containing at least one additive element selected from Bi, V, B, and Cu with the preliminarily baked powder, and firing the mixed powder, and it is characterized that Sr is contained in grain boundary phases existing in the surrounding of the crystal grains of the resulting oxide magnetic material after firing.
As oxides containing the additive elements, those described above with respect to the foregoing oxide magnetic material of the invention can be used.
In the production method of the invention, together with the oxides containing the additive elements, an oxide containing Sr may be mixed with the preliminarily baked powder. The content of Sr in the grain boundary phases can be controlled by controlling the addition amount of the oxide containing Sr. Practically, the content of Sr in the grain boundary phases can be increased by increasing the addition amount.
In the production method of the invention, oxides containing additive elements and/or an oxide containing Sr is added to and mixed with the preliminarily baked powder and in this case, it is generally preferable to carry out the mixing while pulverization by a ball mill or the like being simultaneously carried out. At that time, the conditions for pulverization and mixing are controlled, so that the content of Sr in the grain boundary phases can be controlled. For example, if the mixing duration is prolonged, the content of Sr in the grain boundary phases can be increased.
Also, the content of Sr in the grain boundary phases can be controlled by controlling the temperature for the preliminary baking. Practically, by lowering the preliminary baking temperature is increased the content of Sr in the grain boundary phases.
Further, in the production method of the invention, together with the oxides containing additive elements, an oxide containing Si may be mixed with the preliminarily baked powder. Si can be contained in the grain boundary phases by mixing the oxide containing Si with the preliminarily baked powder.
Also, in the production method of the invention, together with the oxides containing additive elements, an additive, a compound of the Group Ia element or the Group IIa element with a Group VIIb element, may be added to the preliminarily baked powder of the oxide magnetic material. Addition of such an additive to the preliminarily baked powder makes the Group Ia element of the Group IIa element contained in the oxide magnetic material.
The melting point of the additive is preferably 900xc2x0 C. or lower. Practical examples of the additive with a melting point of 900xc2x0 C. or lower include CaCl2 (melting point of 772xc2x0 C.), KF (melting point of 830xc2x0 C.), KI (melting point of 723xc2x0 C.), NaCl (melting point of 800xc2x0 C.), NaI (melting point of 651xc2x0 C.), SrBr2 (melting point of 643xc2x0 C.), SrCl2 (melting point of 873xc2x0 C.), BaBr2 (melting point of 847xc2x0 C.), BaI2 (melting point of 740xc2x0 C.), and the like. Among them, CaCl2 is preferable to be used especially.
The addition amount of the additive is preferably not more than 25% by weight in the preliminarily baked powder. That is, the amount is preferably 33.3 part by weight to 100 part by weight of the preliminarily baked powder. If the addition amount of the additive exceeds 25% by weight, the ratio of the magnetic ceramic material is relatively decreased so that the magnetic properties tend to be deteriorated. The addition amount of the additive is further preferably 0.05 to 25% by weight, furthermore preferably 0.05 to 1% by weight. If the addition amount of the additive is too low, any sufficient effect to provide good magnetic properties by low temperature firing cannot be obtained in some cases.
In the production method of the invention, the mixed powder can be fired after being formed into a substrate-like shape. Accordingly, a magnetic substrate can be produced. As a method to be employed for forming the mixed powder into the substrate-like shape, a method involving adding a binder to the mixed powder, producing a slurry of the resulting powder mixture, and forming a green sheet from the slurry can be exemplified. Further, after a binder is added to the mixed powder, the resulting powder mixture may be press-formed to make the substrate-like shape.
After the green sheet formed into the substrate-like shape is laminated on a substrate-like green sheet produced from another material such as a dielectric material or the like, the obtained laminated body may be fired. Consequently, a laminated ceramic substrate comprising the magnetic substrate and a substrate made of another material such as a dielectric can be obtained.
Wiring patterns can be formed by a screen-printing method or the like on the substrate-like green bodies before firing. Further, via holes and the like can be formed, too.
Since the oxide magnetic material of the invention can be fired at a low firing temperature, a material such as Ag can be used as a material for wiring patterns.